Support integrating as a library about astc-encoder

[solidpixel]

First stage of the API split is implemented.

• All non-uniform globals have been removed.
• API split between front-end and codec added.
• Support for user thread pools added (no automatic threading - decided to remove this).

Currently missing vs the original proposal:

• Non-compressed image layouts are using the original 3D layout, not the zero-copy sliced layout. Split off as #126.
• Build system support. Original plan was to build multiple dynamic libraries (one per ISA), and load a backend on demand. Currently costing up to 7% performance, so want to reduce that before making the change. Split off as #79.

Still needed for 2.0:

• Some higher level API documentation.
• Some documentation on the input image format, and how to get the padding amount needed.

[solidpixel]

Documentation changes merged.